Control units of the above type are known in which the phase voltage is used to generate a sawtooth voltage in dependence on the zero crossing of the phase voltage. The generated sawtooth voltage is then compared with a control voltage and, if in agreement therewith, a firing command for the thyristors is generated. Such a control unit is described, for example, in the German Auslegeschrift No. 2,119,525.
In the aforesaid control units, a problem arises in determining the exact location of the zero crossing of the phase voltage, as this voltage frequently contains harmonics. If the phase voltage is smoothed in order to filter out the harmonics, the smoothing is then accompanied by a phase shift. The better the smoothing, the larger will be the phase shift and, therefore, the displacement of the zero crossing. Since the firing command may have to be given shortly after the zero crossing of the voltage to be controlled, e.g., after 30.degree. el, only relatively little smoothing is possible, if a defined zero crossing is to be obtained prior to the required start of firing. For more smoothing, the zero crossing of another phase must be used in three-phase circuits. The uncertainty in the determination of the zero crossing connected therewith is often undesirable from a control-engineering point of view, especially if highly precise motor controls in machine tool design, for example, are involved.
It is an object of the present invention to provide an improved control unit of the above type.